I just checked the Rossi web site and they indead show the 22-250 as a 1 in 10 twist which seems kinda weard for the 250 , most being a 1-14 or 1-16 . I have the same scope on my H&R 250 and it shoots at point of aim , so i dout that it is the scope .
It may be that the barrel needs a good polishing , also check the crown on the muzzle for anything thats rough or looks not right , something else to look at is trigger pull , having the transfer bar system you will need to pull through the trigger , not just stop when you feel the break .
Single shots can also be very picky about lock-up , so a good cleaning of the lock mec. may be in order , the spray cans of brake cleaner from the auto parts store work very well for this .
Hope this helps .
stimpy